This exploratory study (N = 5,014) of responses to a self-completion questionnaire hypothesized that the proportion of missing values is a ffected b (1) aspects of the questionnaire (item length and content, inventory length and content), and (2) aspects of the responders. Res ponders completed 90.3 % of the items overall. Smokers answered 93.1 % of the requested items, while nonsmokers completed 88.4%. Item len gth was closely related to the number of missing values (r = .78). It ems containing negations were associated with more missing values. A personality inventory also produced more missing values than inventor ies of assumed effects of smoking and of self-efficacy for abstaining from smoking, respectively. High levels of anxiety, anger, depressio n, and low self-respect were related to more missing values. So were very low levels of anger. A multiple stepwise regression analysis sho wed that age was the strongest predictor of missing values (Adj. R2 = .10). Income was inversely related to number of missing values (Adj. R2 for age and income = .19). The �healthy personality� scale had le ss missing items, while the �coronary-prone personality� scale was as sociated with more missing values. An implication of these findings i s that questionnaire items should be kept short and simple. Also, the design of questionnaires should be especially tailored toward specif ic agegroups and sociodemographic subgroups.